This invention provides a process for fabricating oxygen-sensing polymers and methods for measuring the oxygen contents of packages and containers. In particular, this invention provides a process for the preparation and use of flexible polymer films containing an oxygen sensitive indicator and the instrumentation for quantifying said package""s oxygen contents.
There is a need for practical, economical, large-scale manufacturing and implementation of polymeric gas sensors with particular utility to the packaging industry. Packaging of oxygen sensitive foods, pharmaceuticals or medical supplies typically employs engineered gas barrier films and containers, inert gas flushes and high-speed heat-sealing equipment. There are no available technologies that provide a practical means for measuring in-package oxygen on a real time basis. In addition to packaging, many potential applications for oxygen sensing inside containers would benefit from the use of low-cost and sensor-containing barrier polymeric films.
Oxygen monitoring inside containers has required destructive lot testing and laborious, gas-sampling techniques. One common method requires that a needle be inserted into the headspace of a package and an air sample withdrawn for analysis. This method is prone to sampling errors, will result in loss of product (due to package integrity breech) and will provide only historical information rather than real-time data useful for process control. Therefore, there is a need for a non-invasive measurement method.
Optical sensors have been placed into a package and read via changes in absorbance, luminescent intensity or luminescent lifetime properties of the indicators. While these non-invasive optical sensors offer potential advantages over physical sampling techniques, they pose numerous practical disadvantages and added per-test costs to implement. Commonly described optical gas-sensor designs are prepared by casting or painting polymers dissolved in an organic solvent containing gas sensitive optical indicators (e.g., porphyrins, ruthenium, pyrenes) onto a substrate, such as glass, plastic or paper. The sensor can either be prepared as a distinct sensing coupon added to the package prior to sealing or it can be directly painted onto the inside surface of the package.
U.S. Pat. No. 4,810,655 (Khalil) describes an optical sensor cast from a volatile organic solvent solution of indicator and a carrier polymer cast onto a suitable inert substrate. Due to the nature of most suitable organic solvents, direct printing or painting onto flexible packaging using these sensor solutions has the potential for changing the engineered properties of these multilayer films including delamination, cracking, changes in film orientation and migration of processing additives. These problems provide significant hurdles to practical implementation of solvent-based casting, paint or printing methodologies. Other significant problems are inherent to solvent use, such as, flammability of the solvents, hazardous waste storage and removal, and the elimination of toxic fumes for worker safety.
Solventless approaches to preparing sensor films have been described (U.S. Pat. Nos. 4,657,736 and 5,407,829) by casting thin polymerizable films of ruthenium and a silicone copolymer onto food packaging films followed by completing the polymerization to form a sensor element. Although potentially minimizing effects on the packaging film and the other noted problems associated with solvent use, the equipment and logistics of applying and completing polymerization of the sensor material in the volumes necessary for the food packaging industry are well outside the common practice of film manufacturers. One further constraint to xe2x80x9cpaintingxe2x80x9d a sensor onto the overwrap material is the heat-sealing process used in most flexible packaging operations. Packaging films will have a sealing layer designed to provide a bond at specific parameters (e.g., temperature, dwell time) for the packaging machine. Application of even a thin polymer film in the sealing area can result in poor heat seals and package failure. Therefore, any process using xe2x80x9cpaintedxe2x80x9d sensors would require more elaborate film production equipment and orientation of the film during packaging than is commonly found in the industry.
In addition to the difficulty with preparing a practical optical sensor, it also has been difficult to implement an instrument for field use. Lifetime methods provide advantages of internal signal referencing that compensate for changes in signal path and oxygen-sensitive indicator concentration. Two such methods are based on phase-modulation or time-resolved determination of the luminescent lifetime. Although phase-modulation based methods can provide precise measures of lifetime, for applications with low concentrations of oxygen-sensitive indicator and irregular surfaces or sensor configurations with inconsistent reflection properties, the time resolved method provides greater immunity from correlated stray light (excitation) that can lead to non-reproducible performance of the phase-based method.
The addition of pigments to color polymer extrusions has been practiced (e.g., U.S. Pat. No. 5,185,038). Phosphorescent coatings have been prepared using a continuous hot-melt method (U.S. Pat. No. 3,873,390). Both methods use inorganic particles, luminescent or simply colored. In neither case is it suggested that these films would be useful optical gas sensing structures. Also common to patents for pigmenting polymers is the use of inorganic particles, luminescent or simply colored. As noted in Khalil (U.S. Pat. No. 4,801,655), a key element to creating useful luminescent oxygen probes from organic indicators such as porphyrins and chlorins is the complete dissolution of the indicator in the polymer. The thermal melt processes taught by this patent utilize a narrow window of gas-indicator thermal stability and polymer melt processing temperatures common to extrusion processes to form useful oxygen indicating structures.
The present invention provides a process for manufacture of oxygen sensitive polymeric structures by the addition of an oxygen-sensitive indicator directly to the thermal melt phase of polymers during extrusion processes. Preferably, the oxygen-sensitive indicator is luminescent. Importantly, the inventive process allows the integration of oxygen sensing functionality into polymer films, including multilayer barrier films common to the food packaging industry.
The present invention provides a process for making an oxygen-sensitive polymeric structure, comprising
(a) adding an oxygen-sensitive indicator to a thermoplastic polymeric material heated to just above its melting temperature;
(b) mixing the oxygen-sensitive indicator thoroughly within the thermoplastic polymeric material while continuing the heating to maintain the thermoplastic polymeric material in a substantially liquid form; and
(c) forming the mixture into an end product form of indicator-polymer product.
Preferably, the forming step is accomplished by an extrusion process, a molding process or an injection molding process. Preferably, the oxygen-sensitive indicator is selected from the group consisting of polycyclic aromatic hydrocarbons, pyrene, fluoranthene, decacyclene, diphenylanthracene, benzo(g,h,i)perylene), porphyrins, platinum or palladium octaethylporphyrin, tetraphenylporphyrin, tetrabenzporphyrin, chlorins, bacteriochlorins, isobacteriochlorins, chlorophyll), and combinations thereof. Preferably, the polymeric material is selected from the group consisting of linear ethylene alpha olefin copolymers, ethyl vinyl acetate, linear low-density polyethylene (LLDPE), very-low-density polyethylene (VLDPE), metallocene catalyzed polymers, and combinations thereof. Preferably, the melting temperature is from about 140xc2x0 C. to about 240xc2x0 C.
The present invention further provides a multi-layered food packaging film having an ability to detect oxygen presence within a packaging, comprising an indicator polymer product film and a plurality of non-oxygen sensing polymer films bonded thereto, wherein the indicator polymer product is made by a process comprising:
(a) adding an oxygen-sensitive indicator to a thermoplastic polymeric material heated to just above its melting temperature;
(b) mixing the oxygen-sensitive indicator thoroughly within the thermoplastic polymeric material while continuing the heating to maintain the thermoplastic polymeric material in a substantially liquid form; and
(c) forming the mixture into an end product form of indicator-polymer product.
Preferably, the forming step is accomplished by an extrusion process, a molding process or an injection molding process. Preferably, the oxygen-sensitive indicator is selected from the group consisting of polycyclic aromatic hydrocarbons, pyrene, fluoranthene, decacyclene, diphenylanthracene, benzo(g,h,i)perylene), porphyrins, platinum or palladium octaethylporphyrin, tetraphenylporphyrin, tetrabenzporphyrin, chlorins, bacteriochlorins, isobacteriochlorins, chlorophyll), and combinations thereof. Preferably, the polymeric material is selected from the group consisting of linear ethylene alpha olefin copolymers, ethyl vinyl acetate, linear low-density polyethylene (LLDPE), very-low-density polyethylene (VLDPE), metallocene catalyzed polymers, and combinations thereof. Preferably, the melting temperature is from about 140xc2x0 C. to about 240xc2x0 C. Flexible polymeric materials commonly used to form oxygen barrier layers include poly(ethylene vinyl alcohol) (EVOH), polyacrylonitrile, polyvinyl chloride, poly(vinylidene dichloride), polyethylene terephthalate, polyamides, or combinations thereof. Preferably, the indicator polymer product is coextruded or laminated to one or more of these barrier-forming materials during manufacture of the film.